Field of the Invention and Related Art Statement:
This invention relates to a method of machining ceramic rotors for pressure wave type superchargers, and more particularly to a method of machining ends, surfaces, and inner diameters of rotors for pressure wave type superchargers so as to obtain predetermined sizes and dynamic balance of the rotors; with high accuracy.
The ceramic rotors for pressure wave type superchargers have been developed for improving outputs or fuel consumption rates of internal combustion engines, for example, diesel engines.
Referring to FIG. 1, the ceramic rotor 1 for the pressure wave type supercharger (referred to simply as "rotor" hereinafter) has an inner circumferential surface 1e fitted with a rotary shaft (not shown) for rotating the rotor, a slit 1d formed in the inner circumferential surface 1e for fixing the rotary shaft thereto, an outer circumferential surface 1a, end surfaces 1b and 1c, cells 1g for supercharging or supplying the air whose pressure is raised by transmitting high pressure of exhaust gases to the air and an outer wall 1f.
Such a rotor is formed by extruding a ceramic material superior in strength at high temperatures such as silicon nitride by the use of an extruding dies having predetermined through-holes corresponding to the cells shown in FIG. 1 to obtain an extruded honeycombshaped formed body. After the honeycomb formed body has been fired, it is machined by means of a grinding wheel or the like to predetermined dimensions and to obtain a predetermined dynamic balance. This machining is required to have high accuracy with respect to perpendicularity of the end surfaces relative to the inner diameter and concentricity of the inner and outer diameters. Therefore, in holding the rotor for the machining, it has been required to limit a deviation from a proper relation between the end surfaces and outer diameter to the minimum value.
In the machining method of the prior art, the machining of the rotor is effected first on the outer diameter of the outer circumference and parts of the both end surfaces, then the one end surface and the slit, and finally the other end surface and the inner diameter of the inner circumference of the rotor.
In more detail, the machining method of the prior art is carried out in the following manner. In order to machine the outer diameter and the parts of both the end surfaces, as shown in FIG. 2, the rotor 1 is first fixed to a center jig 2 having centers 3, and the outer diameter 1a of the outer circumference of the rotor is ground by a grinding wheel 4 driven by a cylindrical grinder (not shown). Thereafter, parts 1b and 1c of both the end surfaces are ground. In other words, the machining is effected such that a center axis of the outer diameter 1a is brought into alignment with axes of the centers 3 and the parts 1b and 1c form surfaces perpendicular to the axes of the centers 3.
In cutting the end surfaces and slit of the rotor, if it is directly chucked by a scroll chuck generally used in machine tools, the outer wall of the rotor is liable to be broken because a thickness of the outer wall is of the order of 2 mm. In order to avoid such a problem, a chucking method shown in FIG. 3 has been proposed. In this method, after one end of a rotor 1 is adhered into a cylindrical metal die 5 by wax, the die 5 is mounted on a machining apparatus by means of a scroll chuck 6 so that deviations of an outer diameter and an end surface are minimized, which are measured by dial gauges 7. A remaining end surface of the end surface 1b partially machined in the previous step is ground by a grinding wheel 8 and a slit 1d is formed in the rotor 1 by means of a slit grinding wheel 9. The other end surface 1c and an inner diameter 1e of an inner circumferential surface are ground by a cylindrical grinder after the rotor has been chucked in the same manner as described above.
With the above machining method, it is required to adhere the rotor to the metal die for machining one end surface and a slit and the other end surface and an inner diameter. Moreover, centering the rotor is needed, which is a troublesome and time-consuming operation and requires a skillful operator. Further, in order to fulfill the severe requirement in perpendicularity of both end surfaces relative to the inner diameter, machining the one end surface and the slit and the other end surface and the inner diameter must be carried out at some time. Moreover, the length of the metal die directly chucked by the scroll chuck is short and the rotor is supported so as to overhang in a long distance so that a distance between the chucked portion and ground portion of the rotor is long. Therefore, the rotor is apt to move from the chucked position by grinding pressure during grinding so that it is difficult to machine the rotor with high accuracy. Owing to such a difficulty, heavy grinding is impossible and the grinding speed cannot be increased.
Moreover, the machining of outer circumference of the rotor often causes machining scratches or scores. Further, in the event that local deformation of the rotor occurs in a firing step, an outer wall-thickness may become uneven by machining the outer circumference of the rotor so that its mechanical strength decreases.
Moreover, the uneveness in thickness of the outer circumferential wall produced by machining thereat detrimentally affects the dynamic balance of the rotor.